DESCRIPTION (adapted from application abstract): Yellow fever virus (YF) infections are an important and emerging health problem in Africa and South America, despite the availability of the licensed, safe and immunogenic YF 17D strain vaccine. Little is known about the cellular immune responses to YF or the mechanism of pathogenesis in humans. T lymphocyte studies in a related human flavivirus, dengue, have demonstrated that CD4+ and CD8+ memory T cells are generated with the nonstructural NS3 protein as an immunodominant protein. Recently chimaeric flaviviruses have been developed with a YF nonstructural gene as a backbone for Japanese encephalitis (JE) structural genes. These new vaccines will soon enter Phase I clinical trials and further emphasize the importance of learning more about human immune responses to YF. There are two major goals presented training and research in human immune responses to yellow fever virus. To achieve the first goal training a two-year curriculum involving didactic training in biochemistry, molecular biology, cellular and molecular immunology, biostatistics and epidemiology. Laboratory rotations will complement didactic studies in the area of molecular biology. The candidate will learn (1) to construct a vaccines/YF NS3 recombinant virus; (2) to design primers to develop a qualitative nested RT-PCR for the detection of YF RNA in human primate specimens; (3) to create a competitor for the development of a quantitative competitor RT-PCR for the titration of YF RNA; (4) to engineer a YF/dengue chimaeric virus. The molecular reagents developed above will be utilized in the Research Plan. The specific aims are: (1) to define the immunodominant proteins recognized by T lymphocytes in TF- vaccinated individuals; (2) To examine viral replication by quantitative RT- PCR during primary infection/vaccination and its relationship to clinical and/or immunological outcome; (3) To determine if a correlation exists between the pre-existing level of memory cytotoxic T lymphocyte response or neutralizing antibody responses and the development of subsequent viremia, neutralizing antibody responses and/or cellular immune responses following administration of a second dose of YF or chimaeric YF/JE vaccine.